We intend to determine the extent to which DNA of chromosomes binds alkyl and aryl mercury in living cells. Also, we wish to obtain information on the molecular level concerning the nature of the complexes. The method to be used is a spectroscopic one, optical detection of magnetic resonance (ODMR) by means of which magnetic resonance transitions in photoexcited triplet states are detected optically. A heavy atom effect caused by mercury binding to an aromatic chromophore such as a heterocyclic base of DNA makes the resulting triplet state a highly radiative trap which is especially sensitive to ODMR. The ODMR method thus selects the mercury-perturbed chromophore which then can be identified by the magnetic resonance frequencies and other properties of the phosphorescent state. Selectivity by the heavy atom effect is especially good in DNA which normally is not highly luminescent. As a major system for study, we intend to use Allium cepa, for which cytological data of chromosomal aberrations of root cells caused by mercurial treatment exists.